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NATD(8) FreeBSD System Manager's Manual NATD(8)
NAMEnatd -- Network Address Translation daemon
SYNOPSISnatd [-unregistered_only | -u] [-log | -l] [-proxy_only] [-reverse]
[-deny_incoming | -d] [-use_sockets | -s] [-same_ports | -m]
[-verbose | -v] [-dynamic] [-in_port | -iport]
[-out_port | -oport] [-port | -pport]
[-alias_address | -aaddress] [-target_address | -taddress]
[-interface | -ninterface] [-proxy_ruleproxyspec]
[-redirect_portlinkspec] [-redirect_protolinkspec]
[-redirect_addresslinkspec] [-config | -fconfigfile]
[-instanceinstancename] [-globalportport] [-log_denied]
[-log_facilityfacility_name] [-punch_fwfirewall_range]
[-skinny_portport] [-log_ipfw_denied] [-pid_file | -Ppidfile]
[-exit_delay | -Pms]
DESCRIPTION
The natd utility provides a Network Address Translation facility for use
with divert(4) sockets under FreeBSD.
(If you need NAT on a PPP link, ppp(8) provides the -nat option that
gives most of the natd functionality, and uses the same libalias(3)
library.)
The natd utility normally runs in the background as a daemon. It is
passed raw IP packets as they travel into and out of the machine, and
will possibly change these before re-injecting them back into the IP
packet stream.
It changes all packets destined for another host so that their source IP
address is that of the current machine. For each packet changed in this
manner, an internal table entry is created to record this fact. The
source port number is also changed to indicate the table entry applying
to the packet. Packets that are received with a target IP of the current
host are checked against this internal table. If an entry is found, it
is used to determine the correct target IP address and port to place in
the packet.
The following command line options are available:
-log | -l Log various aliasing statistics and information to the file
/var/log/alias.log. This file is truncated each time natd is
started.
-deny_incoming | -d
Do not pass incoming packets that have no entry in the inter-
nal translation table.
If this option is not used, then such a packet will be
altered using the rules in -target_address below, and the
entry will be made in the internal translation table.
-log_denied
Log denied incoming packets via syslog(3) (see also
-log_facility).
-log_facilityfacility_name
Use specified log facility when logging information via
syslog(3). Argument facility_name is one of the keywords
specified in syslog.conf(5).
-use_sockets | -s
Allocate a socket(2) in order to establish an FTP data or IRC
DCC send connection. This option uses more system resources,
but guarantees successful connections when port numbers con-
flict.
-same_ports | -m
Try to keep the same port number when altering outgoing pack-
ets. With this option, protocols such as RPC will have a
better chance of working. If it is not possible to maintain
the port number, it will be silently changed as per normal.
-verbose | -v
Do not call daemon(3) on startup. Instead, stay attached to
the controlling terminal and display all packet alterations
to the standard output. This option should only be used for
debugging purposes.
-unregistered_only | -u
Only alter outgoing packets with an unregistered source
address. According to RFC 1918, unregistered source
addresses are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.
-redirect_portprototargetIP:targetPORT[-targetPORT]
[aliasIP:]aliasPORT[-aliasPORT]
[remoteIP[:remotePORT[-remotePORT]]]
Redirect incoming connections arriving to given port(s) to
another host and port(s). Argument proto is either tcp or
udp, targetIP is the desired target IP address, targetPORT is
the desired target port number or range, aliasPORT is the
requested port number or range, and aliasIP is the aliasing
address. Arguments remoteIP and remotePORT can be used to
specify the connection more accurately if necessary. If
remotePORT is not specified, it is assumed to be all ports.
Arguments targetIP, aliasIP and remoteIP can be given as IP
addresses or as hostnames. The targetPORT, aliasPORT and
remotePORT ranges need not be the same numerically, but must
have the same size. When targetPORT, aliasPORT or remotePORT
specifies a singular value (not a range), it can be given as
a service name that is searched for in the services(5) data-
base.
For example, the argument
tcpinside1:telnet6666
means that incoming TCP packets destined for port 6666 on
this machine will be sent to the telnet port on the inside1
machine.
tcpinside2:2300-23993300-3399
will redirect incoming connections on ports 3300-3399 to host
inside2, ports 2300-2399. The mapping is 1:1 meaning port
3300 maps to 2300, 3301 maps to 2301, etc.
-redirect_protoprotolocalIP [publicIP [remoteIP]]
Redirect incoming IP packets of protocol proto (see
protocols(5)) destined for publicIP address to a localIP
address and vice versa.
If publicIP is not specified, then the default aliasing
address is used. If remoteIP is specified, then only packets
coming from/to remoteIP will match the rule.
-redirect_addresslocalIPpublicIP
Redirect traffic for public IP address to a machine on the
local network. This function is known as staticNAT. Nor-
mally static NAT is useful if your ISP has allocated a small
block of IP addresses to you, but it can even be used in the
case of single address:
redirect_address10.0.0.80.0.0.0
The above command would redirect all incoming traffic to
machine 10.0.0.8.
If several address aliases specify the same public address as
follows
redirect_address 192.168.0.2 public_addr
redirect_address 192.168.0.3 public_addr
redirect_address 192.168.0.4 public_addr
the incoming traffic will be directed to the last translated
local address (192.168.0.4), but outgoing traffic from the
first two addresses will still be aliased to appear from the
specified public_addr.
-redirect_portprototargetIP:targetPORT[,targetIP:targetPORT[,...]]
[aliasIP:]aliasPORT [remoteIP[:remotePORT]]
-redirect_addresslocalIP[,localIP[,...]] publicIP
These forms of -redirect_port and -redirect_address are used
to transparently offload network load on a single server and
distribute the load across a pool of servers. This function
is known as LSNAT (RFC 2391). For example, the argument
tcpwww1:http,www2:http,www3:httpwww:http
means that incoming HTTP requests for host www will be trans-
parently redirected to one of the www1, www2 or www3, where a
host is selected simply on a round-robin basis, without
regard to load on the net.
-dynamic If the -n or -interface option is used, natd will monitor the
routing socket for alterations to the interface passed. If
the interface's IP address is changed, natd will dynamically
alter its concept of the alias address.
-in_port | -iport
Read from and write to divert(4) port port, treating all
packets as ``incoming''.
-out_port | -oport
Read from and write to divert(4) port port, treating all
packets as ``outgoing''.
-port | -pport
Read from and write to divert(4) port port, distinguishing
packets as ``incoming'' or ``outgoing'' using the rules spec-
ified in divert(4). If port is not numeric, it is searched
for in the services(5) database. If this option is not spec-
ified, the divert port named natd will be used as a default.
-alias_address | -aaddress
Use address as the aliasing address. Either this or the
-interface option must be used (but not both), if the
-proxy_only option is not specified. The specified address
is usually the address assigned to the ``public'' network
interface.
All data passing out will be rewritten with a source address
equal to address. All data coming in will be checked to see
if it matches any already-aliased outgoing connection. If it
does, the packet is altered accordingly. If not, all
-redirect_port, -redirect_proto and -redirect_address assign-
ments are checked and actioned. If no other action can be
made and if -deny_incoming is not specified, the packet is
delivered to the local machine using the rules specified in
-target_address option below.
-t | -target_addressaddress
Set the target address. When an incoming packet not associ-
ated with any pre-existing link arrives at the host machine,
it will be sent to the specified address.
The target address may be set to 255.255.255.255, in which
case all new incoming packets go to the alias address set by
-alias_address or -interface.
If this option is not used, or called with the argument
0.0.0.0, then all new incoming packets go to the address
specified in the packet. This allows external machines to
talk directly to internal machines if they can route packets
to the machine in question.
-interface | -ninterface
Use interface to determine the aliasing address. If there is
a possibility that the IP address associated with interface
may change, the -dynamic option should also be used. If this
option is not specified, the -alias_address option must be
used.
The specified interface is usually the ``public'' (or
``external'') network interface.
-config | -ffile
Read configuration from file. A file should contain a list
of options, one per line, in the same form as the long form
of the above command line options. For example, the line
alias_address 158.152.17.1
would specify an alias address of 158.152.17.1. Options that
do not take an argument are specified with an argument of yes
or no in the configuration file. For example, the line
log yes
is synonymous with -log.
Options can be divided to several sections. Each section
applies to own natd instance. This ability allows to config-
ure one natd process for several NAT instances. The first
instance that always exists is a "default" instance. Each
another instance should begin with
instance instance_name
At the next should be placed a configuration option. Exam-
ple:
# default instance
port 8668
alias_address 158.152.17.1
# second instance
instance dsl1
port 8888
alias_address 192.168.0.1
Trailing spaces and empty lines are ignored. A `#' sign will
mark the rest of the line as a comment.
-instanceinstancename
This option switches command line options processing to con-
figure instance instancename (creating it if necessary) till
the next -instance option or end of command line. It is eas-
ier to set up multiple instances in the configuration file
specified with the -config option rather than on a command
line.
-globalportport
Read from and write to divert(4) port port, treating all
packets as ``outgoing''. This option is intended to be used
with multiple instances: packets received on this port are
checked against internal translation tables of every config-
ured instance. If an entry is found, packet is aliased
according to that entry. In no entry was found in any of the
instances, packet is passed unchanged, and no new entry will
be created. See the section MULTIPLEINSTANCES for more
details.
-reverse This option makes natd reverse the way it handles
``incoming'' and ``outgoing'' packets, allowing it to operate
on the ``internal'' network interface rather than the
``external'' one.
This can be useful in some transparent proxying situations
when outgoing traffic is redirected to the local machine and
natd is running on the internal interface (it usually runs on
the external interface).
-proxy_only
Force natd to perform transparent proxying only. Normal
address translation is not performed.
-proxy_rule [typeencode_ip_hdr | encode_tcp_stream] portxxxxservera.b.c.d:yyyy
Enable transparent proxying. Outgoing TCP packets with the
given port going through this host to any other host are
redirected to the given server and port. Optionally, the
original target address can be encoded into the packet. Use
encode_ip_hdr to put this information into the IP option
field or encode_tcp_stream to inject the data into the begin-
ning of the TCP stream.
-punch_fwbasenumber:count
This option directs natd to ``punch holes'' in an
ipfirewall(4) based firewall for FTP/IRC DCC connections.
This is done dynamically by installing temporary firewall
rules which allow a particular connection (and only that con-
nection) to go through the firewall. The rules are removed
once the corresponding connection terminates.
A maximum of count rules starting from the rule number
basenumber will be used for punching firewall holes. The
range will be cleared for all rules on startup. This option
has no effect when the kernel is in security level 3, see
init(8) for more information.
-skinny_portport
This option allows you to specify the TCP port used for the
Skinny Station protocol. Skinny is used by Cisco IP phones
to communicate with Cisco Call Managers to set up voice over
IP calls. By default, Skinny aliasing is not performed. The
typical port value for Skinny is 2000.
-log_ipfw_denied
Log when a packet cannot be re-injected because an ipfw(8)
rule blocks it. This is the default with -verbose.
-pid_file | -Pfile
Specify an alternate file in which to store the process ID.
The default is /var/run/natd.pid.
-exit_delayms
Specify delay in ms before daemon exit after signal. The
default is 10000.
RUNNING NATD
The following steps are necessary before attempting to run natd:
1. Build a custom kernel with the following options:
options IPFIREWALL
options IPDIVERT
Refer to the handbook for detailed instructions on building a custom
kernel.
2. Ensure that your machine is acting as a gateway. This can be done
by specifying the line
gateway_enable=YES
in the /etc/rc.conf file or using the command
sysctl net.inet.ip.forwarding=1
3. If you use the -interface option, make sure that your interface is
already configured. If, for example, you wish to specify `tun0' as
your interface, and you are using ppp(8) on that interface, you must
make sure that you start ppp prior to starting natd.
Running natd is fairly straight forward. The line
natd -interface ed0
should suffice in most cases (substituting the correct interface name).
Please check rc.conf(5) on how to configure it to be started automati-
cally during boot. Once natd is running, you must ensure that traffic is
diverted to natd:
1. You will need to adjust the /etc/rc.firewall script to taste. If
you are not interested in having a firewall, the following lines
will do:
/sbin/ipfw -f flush
/sbin/ipfw add divert natd all from any to any via ed0
/sbin/ipfw add pass all from any to any
The second line depends on your interface (change `ed0' as appropri-
ate).
You should be aware of the fact that, with these firewall settings,
everyone on your local network can fake his source-address using
your host as gateway. If there are other hosts on your local net-
work, you are strongly encouraged to create firewall rules that only
allow traffic to and from trusted hosts.
If you specify real firewall rules, it is best to specify line 2 at
the start of the script so that natd sees all packets before they
are dropped by the firewall.
After translation by natd, packets re-enter the firewall at the rule
number following the rule number that caused the diversion (not the
next rule if there are several at the same number).
2. Enable your firewall by setting
firewall_enable=YES
in /etc/rc.conf. This tells the system startup scripts to run the
/etc/rc.firewall script. If you do not wish to reboot now, just run
this by hand from the console. NEVER run this from a remote session
unless you put it into the background. If you do, you will lock
yourself out after the flush takes place, and execution of
/etc/rc.firewall will stop at this point - blocking all accesses
permanently. Running the script in the background should be enough
to prevent this disaster.
MULTIPLE INSTANCES
It is not so uncommon to have a need of aliasing to several external IP
addresses. While this traditionally was achieved by running several natd
processes with independent configurations, natd can have multiple alias-
ing instances in a single process, also allowing them to be not so inde-
pendent of each other. For example, let us see a common task of load
balancing two channels to different providers on a machine with two
external interfaces `sis0' (with IP 1.2.3.4) and `sis2' (with IP
2.3.4.5):
net 1.2.3.0/24
1.2.3.1 ------------------ sis0
(router) (1.2.3.4)
net 10.0.0.0/24
sis1 ------------------- 10.0.0.2
(10.0.0.1)
net 2.3.4.0/24
2.3.4.1 ------------------ sis2
(router) (2.3.4.5)
Default route is out via `sis0'.
Interior machine (10.0.0.2) is accessible on TCP port 122 through both
exterior IPs, and outgoing connections choose a path randomly between
`sis0' and `sis2'.
The way this works is that natd.conf builds two instances of the aliasing
engine.
In addition to these instances' private divert(4) sockets, a third socket
called the ``globalport'' is created; packets sent to natd via this one
will be matched against all instances and translated if an existing entry
is found, and unchanged if no entry is found. The following lines are
placed into /etc/natd.conf:
log
deny_incoming
verbose
instance default
interface sis0
port 1000
redirect_port tcp 10.0.0.2:122 122
instance sis2
interface sis2
port 2000
redirect_port tcp 10.0.0.2:122 122
globalport 3000
And the following ipfw(8) rules are used:
ipfw -f flush
ipfw add allow ip from any to any via sis1
ipfw add skipto 1000 ip from any to any in via sis0
ipfw add skipto 2000 ip from any to any out via sis0
ipfw add skipto 3000 ip from any to any in via sis2
ipfw add skipto 4000 ip from any to any out via sis2
ipfw add 1000 count ip from any to any
ipfw add divert 1000 ip from any to any
ipfw add allow ip from any to any
ipfw add 2000 count ip from any to any
ipfw add divert 3000 ip from any to any
ipfw add allow ip from 1.2.3.4 to any
ipfw add skipto 5000 ip from 2.3.4.5 to any
ipfw add prob .5 skipto 4000 ip from any to any
ipfw add divert 1000 ip from any to any
ipfw add allow ip from any to any
ipfw add 3000 count ip from any to any
ipfw add divert 2000 ip from any to any
ipfw add allow ip from any to any
ipfw add 4000 count ip from any to any
ipfw add divert 2000 ip from any to any
ipfw add 5000 fwd 2.3.4.1 ip from 2.3.4.5 to not 2.3.4.0/24
ipfw add allow ip from any to any
Here the packet from internal network to Internet goes out via `sis0'
(rule number 2000) and gets catched by the globalport socket (3000).
After that, either a match is found in a translation table of one of the
two instances, or the packet is passed to one of the two other divert(4)
ports (1000 or 2000), with equal probability. This ensures that load
balancing is done on a per-flow basis (i.e., packets from a single TCP
connection always flow through the same interface). Translated packets
with source IP of a non-default interface (`sis2') are forwarded to the
appropriate router on that interface.
SEE ALSOlibalias(3), divert(4), protocols(5), rc.conf(5), services(5),
syslog.conf(5), init(8), ipfw(8), ppp(8)AUTHORS
This program is the result of the efforts of many people at different
times:
Archie Cobbs <archie@FreeBSD.org> (divert sockets)
Charles Mott <cm@linktel.net> (packet aliasing)
Eivind Eklund <perhaps@yes.no> (IRC support & misc additions)
Ari Suutari <suutari@iki.fi> (natd)
Dru Nelson <dnelson@redwoodsoft.com> (early PPTP support)
Brian Somers <brian@awfulhak.org> (glue)
Ruslan Ermilov <ru@FreeBSD.org> (natd, packet aliasing, glue)
Poul-Henning Kamp <phk@FreeBSD.org> (multiple instances)
FreeBSD 10.1 June 23, 2008 FreeBSD 10.1